1. Field of the Invention
This invention relates to polycrystalline transparent spinel sintered body and a method of producing the same.
2. Description of the Prior Art
It is known that spinel oxides containing Al.sub.2 O.sub.3 and MgO have a crystal structure corresponding to the cubic system and they are free from double refraction, and that when a polycrystalline spinel sintered body is formed therefrom, no light scattering occurs at grain boundaries. Therefore, if the amounts of impurities and pores therein are kept low, a polycrystalline transparent spinel sintered body having a high in-line transmittance can be obtained by using spinel oxides. Three types of polycrystalline transparent spinel sintered bodies of the prior art are known, namely:
(1) a sintered body consisting of Al.sub.2 O.sub.3 and MgO at a molar ratio 0.50:0.50 (to be referred to as the "equimolar composition", hereinafter); PA1 (2) a sintered body containing MgO in excess of the equimolar composition; and PA1 (3) a sintered body made by adding one or more sintering aids, such as calcium oxide (CaO) and lithium fluoride (LiF).
The above-mentioned sintered body (1) with the equimolar composition has a shortcoming in that when it is made under non-pressurized conditions it is not dense and its transmittance is low, so that in order to obtain a transparent sintered body, sintering at an elevated pressure, i.e., hot pressing, is necessary. This results in a comparatively low productivity and a high cost. The sintered body (2) containing MgO in excess of the equimolar composition has a shortcoming in that it is difficult to combine the excess MgO in the spinel in the form of a solid solution but tends to form residual second phase therein, resulting in a low transmittance of light. The sintered body (3) with the sintering aid has a shortcoming in that the sintering aid also tends to form a residual second phase in the sintered body so as to reduce the transmittance thereof.